The P0306 is one of several misfire codes you can read on your scan tool. I studied 120+ different vehicles with misfires and how were they fixed, to provide you with a list of the most common causes, also with an action plan to diagnose this fault in no time.
You don’t want to delay fixing your P0306 as the misfire will damage your catalytic converters or engine internals. Both of these fixes are priced at thousands of dollars while the misfire is often caused by a worn-out $5 spark plug. Fix it ASAP.
P0306 quick overview
| Meaning | P0306: Cylinder #6 Misfire Detected |
| Is it serious? | Yes, this code is serious and should be fixed as soon as possible to avoid further damage to the engine or catalytic converter. |
| Possible causes | -Worn or faulty spark plug in cylinder #6 -Defective ignition coil for cylinder #6 -Malfunctioning fuel injector for cylinder #6 -Timing issues (e.g., damaged crankshaft pulley or gear) -Internal engine issues (e.g., valve problems, piston ring wear) -Wiring or electrical connection problems related to cylinder #6 |
| How to diagnose? | Inspect Spark Plug: Check the spark plug in cylinder #6 for signs of wear or fouling. Test Ignition Coil: Swap the ignition coil with another cylinder to see if the misfire follows. Check Fuel Injector: Perform a swap or test of the fuel injector in cylinder #6 to ensure it’s functioning correctly. Perform Compression Test: Conduct a compression test on cylinder #6 to rule out internal mechanical issues. Inspect Timing Components: Examine the timing belt, crankshaft pulley, and related components for any damage. Inspect Wiring: Examine wiring and connections related to cylinder #6 for any faults or loose connections. |
P0306 Meaning
The P0306 code shows that your car’s engine isn’t burning fuel properly in cylinder #6. This can cause the engine to run poorly and might lead to more expensive damage if not fixed soon. Start by checking the spark plug, ignition coil, and fuel injector in that cylinder.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
Case study: real cases of solving P0306
I checked specifically for code P0306 and picked these example fixes from my full case study of 123 vehicles with misfires.
How to fix P0306 on Lexus
Car: 2002 Lexus IS300 3.0L
Problem: P0301, P0306, Cuts out, Misfire, MIL Lamp on
Diagnosing: The technician replaced a bunch of parts from the timing belt to spark plugs and PCM but all without success. The car didn’t show any other codes, but scoping the cam and crank sensors showed differences and possibly a timing problem.
Fix: Fix the broken crankshaft gear
How to fix P0306 on FORD
Car: 2000 Ford F-150 5.4L
Problem: Misfire, P0301, P0306 Excessive Emissions, Poor Idle Quality
Diagnosing: Checking the fuel trims pointed the technician in the right direction. When the misfire happened the fuel trim on one bank stayed normal, but the other displayed -25% indicating rich condition in this bank, possibly an ignition issue.
Fix: Fixed by changing the spark plugs and ignition coils.
How to fix P0306 on Toyota
Car: 1997 Toyota 4Runner SR5 3.4L
Problem: Misfire, P0300, P0301 – P0304, P0306
Diagnosing: The technician tried to find a fault in the live data (mode $01), but without any success. The fault was hiding in the timing.
Fix: Replacing the damaged crankshaft pulley.
How to fix P0306 on Hyundai
Car: 2000 Hyundai Sonata GLS 2.5L
Problem: Misfire, P0301 – P0306, Cuts out, MIL Lamp on
Diagnosing: The technician received the vehicle after another shop couldn’t fix it after changing almost everything in the ignition, fuel + timing, and other parts. Even the scope couldn’t find anything but he noticed a strange sound at 2000 RPM, which went away after removing the serpentine belt.
Fix: Replace serpentine belts and pulleys.
How to fix P0306 on BMW
Car: 2001 BMW X5 3.0i
Problem: Misfire, MIL Lamp on, P0301 – P0306
Diagnosing: The technician changed all spark plugs and ignition coils without success. The smoke test detected a small vacuum leak.
Fix: Repair vacuum leak
The key takeaways from these cases
- Negative fuel trims mean it is trying to correct for a rich condition. Maybe the coolant temperature sensor is sending false info to the PCM. May also be a bad map sensor.
- Remember that new aftermarket (even OEM) parts can come faulty. Don’t forget that in your diagnosing process
P0306 FAQ and conclusion:
Check the Wikipedia article on combustion engines if you are still unsure what exactly the misfire is. Understanding it will help you with your diagnosis.
The key takeaways from these cases
- Don’t change coils/spark plugs unless you confirm they are faulty (do a swap test at least)
- A lot of misfire cases are caused by internal engine damage/timing so don’t rush with replacing ignition/fuel parts without further diagnosis
- Your diagnosing is sometimes right, but only the OEM part will solve the issue
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko